The direct methanol fuel cell (DMFC) catalytically oxidizes methanol to generate electricity. The DMFC differs from PEM (proton exchange membrane) or solid polymer fuel cells, which use hydrogen gas for generating electricity. One major advantage of the DMFC over the PEM fuel cell is its ability to use methanol, a relatively inexpensive and easily handled material when compared to hydrogen gas. One major disadvantage of the DMFC, when compared to the PEM fuel cell, is methanol crossover. Methanol crossover occurs when methanol from the anode crosses to the cathode. This causes the loss of efficiency of the cell. Nevertheless, the DMFC appears to be a viable portable power source for devices such as cellular or mobile telephones, and handheld or laptop computers. “Types of Fuel Cells,” Fuel Cells 2000, www.fuelcells.org; Thomas, et al, “Fuel Cells-Green Power,” Los Alamos National Laboratory, LA-VR-99-3231.
The DMFC is an electrochemical device. The anodic catalyzed reaction is:CH3OH+H2O→CO2+6H++6e−
The cathodic catalyzed reaction is:3/2 O2+6H++6e−→3H2OThe overall cell reaction is:CH3OH+3/2 O2→CO2+2H2O
These cells operate at efficiencies of about 40% at temperatures of 50-100° C., the efficiencies will increase at higher operating temperatures. Fuel Cells 2000, Ibid; Thomas, Ibid.
As with any chemical reaction, reactants, products, and unwanted products (by-products) become mixed as the reaction proceeds, and separation of these materials is an engineering challenge. So, at the anode, methanol, water, and carbon dioxide will be mixed together. One must be careful that excess methanol not accumulate at the anode because it will crossover the proton conducting membrane (PCM) and decrease the cell's efficiency. Water is good for the PCM, which needs water to maintain its proton conductivity, but if water accumulates, it can prevent methanol from reaching the catalyst, or it can be recycled back into fuel mixture where it can dilute the fuel. Both can decrease the efficiency of the cell. Carbon dioxide (or COxs) must be removed to allow room for the fuel at the anode. Otherwise, cell efficiency can suffer.
Likewise, at the cathode, oxygen typically from air, must reach the cathode and water must be removed. If oxygen cannot reach the cathode, efficiency drops because the cathode half cell reaction is impeded. If water, which can be used to moisten the PCM, is allowed to accumulate, it will prevent oxygen from reaching the cathode.
One challenge related to the foregoing is managing the reactant/product issues without greatly increasing the size or weight of the DMFC. DMFC is targeted, in part, at a portable power source for cellular or mobile telephones and handheld or laptop computers.
In WO 02/45196 A2, a DMFC is disclosed. Referring to FIG. 3, the DMFC 40 has proton conducting membrane (PCM) 80 with CO2 conducting elements 52. On the anode side 41, there is a conducting plate 23 that has a flow field 25, a gas diffusion layer 44, and an anodic catalyst 42. On the cathode side 31, there is a conducting plate 33 with a flow field 35, a gas diffusion layer 48, and a cathode catalyst 46. The catalyst, anode or cathode, is applied to either a surface of the PCM 80 or to the gas diffusion layers 44, 48. The respective flow fields are in communication with their respective gas diffusion layers and the combined action of these flow fields and diffusion layers is intended to ensure the even distribution of reactants to the catalyst and the efficient removal of unwanted products, by-products, and unreacted reactants for the reaction. The gas diffusion layers are made of carbon fiber paper and/or carbon fiber cloth and may be “wet-proofed” with PTFE polymer. Note that the gas diffusion layer, catalyst, and PCM are in close contact to promote electrons or protons conductivity.
On the anode side, fuel (methanol, methanol/water in either liquid or vapor form) is introduced at one end of the flow field 25, and by products (water, CO2, and un-reacted fuel) are removed at other end of the flow field 25. CO2 produced at the anode is intended to cross the PCM 80 via CO2 conductors 52. Water produced at the anode is not meant to remain in the gas diffusion layer 42 as is apparent from the use of the PTFE. On the cathode side, air (the source of O2) is introduced at one end of flow field 35, and water, unreacted air, and CO2 are removed at the other end of flow field 35. Water produced at the cathode is not intended to remain in the gas diffusion layer 48 as is apparent from the use of the PTFE.
In U.S. patent application Publication 2002/0192537 A1, another DMFC is disclosed. This DMFC is similar to the foregoing DMFC, except the carbon paper or carbon cloth gas diffusion layers are replaced with a porous metal layer. See paragraphs [0022-0024].
Accordingly, there is a need to improve reactant, product, and by-product management at both the anode and cathode of DMFC while not significantly increasing the size or weight of the DMFC.